The present invention relates generally to proton exchange membranes of the type suitable for use in electrochemical devices, such as fuel cells, and relates more particularly to a novel proton exchange membrane.
Fuel cells are electrochemical devices in which a free energy change resulting from a fuel oxidation reaction is converted into electrical energy. Because of their comparatively high inherent efficiencies and comparatively low emissions, fuel cells are presently receiving considerable attention as a possible alternative to the combustion of nonrenewable fossil fuels in a variety of applications.
A typical fuel cell comprises a fuel electrode (i.e., anode) and an oxidant electrode (i.e., cathode), the two electrodes being separated by an electrolyte that is a good conductor of ions but a poor conductor of electrons. The electrodes are connected electrically to a load, such as an electronic circuit, by an external circuit conductor. Oxidation of the fuel at the anode produces electrons that flow through the external circuit to the cathode producing an electric current. The electrons react with an oxidant at the cathode. In theory, any substance capable of chemical oxidation that can be supplied continuously to the anode can serve as the fuel for the fuel cell, and any material that can be reduced at a sufficient rate at the cathode can serve as the oxidant for the fuel cell.
In one well-known type of fuel cell, sometimes referred to as a hydrogen fuel cell, gaseous hydrogen serves as the fuel, and gaseous oxygen, which is typically supplied from the air, serves as the oxidant. The electrodes in a hydrogen fuel cell are typically porous to permit the gas-electrolyte junction to be as great as possible. At the anode, incoming hydrogen gas ionizes to produce hydrogen ions and electrons. Since the electrolyte is a non-electronic conductor, the electrons flow away from the anode via the external circuit, producing an electric current. At the cathode, oxygen gas reacts with hydrogen ions migrating through the electrolyte and the incoming electrons from the external circuit to produce water as a byproduct. The overall reaction that takes place in the fuel cell is the sum of the anode and cathode reactions, with part of the free energy of reaction being released directly as electrical energy and with another part of the free energy being released as heat at the fuel cell.
In another well-known type of fuel cell, sometimes referred to as a direct organic fuel cell, an organic fuel is oxidized at the anode. One of the more common organic fuels is methanol although ethanol, propanol, isopropanol, trimethoxymethane, dimethoxymethane, dimethyl ether, trioxane, formaldehyde, and formic acid are also suitable for use. During operation of a typically direct methanol fuel cell, a mixture of methanol and water is circulated over the anode. The circulation of the methanol/water mixture over the anode causes electrons to be released in the following electrochemical reaction:Anode: CH3OH+H2O→CO2+6H++6e−  (1)
Carbon dioxide produced by the above reaction is then discharged from the fuel cell, together with any excess methanol/water mixture. (The carbon dioxide is then typically separated from the methanol/water mixture, and the methanol/water mixture is then typically re-circulated to the anode using a pump.) At the same time the electrochemical reaction described in equation (1) above is occurring, gaseous oxygen (or air) is circulated over the cathode. The circulation of oxygen over the cathode causes electrons to be captured in the following electrochemical reaction:Cathode: 1.5O2+6H++6e−→3H2O  (2)
Excess oxygen (or air) and water are then discharged from the fuel cell. (The water may be recovered from the effluent air stream by a water/gas separator and/or by a condenser.) The individual electrode reactions described by equations (1) and (2) result in the following overall reaction for the fuel cell, with a concomitant flow of electrons:Overall: CH3OH+1.5O2→CO2+2H2O  (3)
Although the electrolyte of a fuel cell may be a liquid electrolyte, more commonly the electrolyte of a fuel cell is a solid polymer electrolyte or proton exchange membrane (PEM). The advantages of using a PEM, as opposed to a liquid electrolyte, in a fuel cell are numerous. For example, PEMs are simpler and more compact than most liquid electrolytes. In addition, the use of a PEM, instead of a liquid electrolyte, simplifies fluid management and eliminates the potential of corrosive liquids. Furthermore, fuel cells containing PEMs are capable of being operated at temperatures close to room temperature (typically around 80° C.) whereas fuel cells containing liquid electrolytes typically must be operated at temperatures far exceeding room temperature. One of the more common types of PEMs is a perfluorosulfonic acid (PFSA) polymer, said PFSA polymer being formed by the copolymerization of tetrafluoroethylene and perfluorovinylether sulfonic acid. See e.g., U.S. Pat. No. 3,282,875, inventors Connolly et al., issued Nov. 1, 1966; U.S. Pat. No. 4,470,889, inventors Ezzell et al., issued Sep. 11, 1984; U.S. Pat. No. 4,478,695, inventors Ezzell et al., issued Oct. 23, 1984; U.S. Pat. No. 6,492,431, inventor Cisar, issued Dec. 10, 2002, all of which are incorporated herein by reference. A commercial embodiment of a perfluorosulfonic acid polymer PEM is available from DuPont (Wilmington, Del.) as NAFION® PFSA polymer.
Although proton exchange membranes and, in particular, PFSA proton exchange membranes are generally satisfactory as the electrolyte of a fuel cell, there nonetheless remains room for improvement in certain properties of PEMs. For example, one common difficulty associated with PEMs is that PEMs have a tendency to tear, especially when being handled (as is the case during assembly of a fuel cell) or in stressed areas where compression is applied thereto (as is the case in peripheral areas of PEMs sealed under pressure to other fuel cell components). Because the tendency to tear is greatest when PEMs are wet and because PEMs must be wet in order to function properly, one approach to this problem has been to assemble fuel cells with dry PEMs and then to subject the PEMs to a humidification process. This approach, however, has its own shortcomings. One such shortcoming is that the dry assembly requires special moisture-free facilities, such as a “dry room.” Another such shortcoming is that the humidification process is time-consuming. Still another such shortcoming is that the humidification process typically results in the PEM swelling in a non-uniform manner, thereby creating stress in some areas of the PEM and introducing irregularities in the contact pressure applied over the entire active surface area of the PEM. (When the contact pressure is not uniform over the entire active surface area of the PEM, the performance of the fuel cell is adversely affected.) As can readily be appreciated, such irregularities are amplified where humidification is applied to a plurality of PEM-containing fuel cells arranged in a stack.
Another common difficulty associated with PEMs is that PEMs have a tendency to be permeable to gases and water. Such permeability is undesirable as it may result in un-oxidized fuel entering the PEM and then escaping from the fuel cell through the peripheral edges of the PEM, thereby resulting in fuel loss (and, in the case of some fuels like hydrogen gas, in the escape of a highly combustible gas), and/or may result in water leaking from the PEM, thereby degrading PEM performance. One of the approaches to addressing this problem of leakage or permeability is compressing, under great pressure, the peripheral edges of the PEM between a pair of silicone gaskets, which are, in turn, compressed under great pressure between the edges of a pair of rigid, non-porous, conductive substrates patterned with flow fields. (In some cases, the mating faces of the PEM are additionally machined or molded into ridges to facilitate compression of the PEM.) The foregoing approach, however, is inadequate to address sufficiently the problems of fuel loss and water leakage. Moreover, as explained above, the high pressure used to compress the PEM can cause the PEM to be physically stressed to an extent where tearing is more likely. As can readily be appreciated, the tearing of the PEM is likely to result in the additional loss of gases and water from the PEM.
Still another common difficulty associated with PEMs, particularly strongly acidic PEMs like perfluorosulfonic acid (PFSA) PEMs, is that there is a tendency for the acidic PEMs to react chemically with the silicone gaskets contacted therewith. Such a chemical reaction results in the degradation of the silicone gaskets and in the contamination of the PEM, both results being highly undesirable.
Often, a number of fuel cells are assembled together in order to meet desired voltage and current requirements. One common type of assembly, often referred to as a bipolar stack, comprises a plurality of stacked fuel cells that are electrically connected in series in a bipolar configuration. Another common type of assembly, often referred to as a segmented fuel cell or planar fuel cell, comprises two or more sets of electrodes contacted with a common PEM, the electrode sets being separated by uncatalyzed border areas. Each electrode set bonded to the common PEM comprises a cell. The cells are then connected in series or in parallel to achieve a desired voltage and current. As can readily be appreciated, a segmented fuel cell has the advantage of being more compact than a bipolar stack. In addition, a segmented fuel cell permits designs which form on planar or curved surfaces. Unfortunately, however, the progress of segmented fuel cells has been hampered by the presence of mobile protons and water in the uncatalyzed border areas, which promotes the occurrence of electrolytic leakage paths between the catalyzed areas or cells, and by the crossover, in the uncatalyzed border areas, of gas reactants from one cell to another cell.